When mobile phones are in a standby or idle mode, the idle mobile phones must listen to a signal radiated by the communication system in order to recognize if and when the mobile phone is being called. An attractive communication system from the user's viewpoint would be a dual mode satellite/cellular telephone that would listen to and connect calls with a land-based cellular system if the mobile phone is in range, or to a satellite system when the land-based cellular system is not in range. The principle advantage of such a dual system is that the number of subscribers outside cellular coverage at any instant might only be a small fraction of the total number of subscribers, thus the number of subscribers needing to access the satellite system is reduced. This permits a satellite system of limited capacity to take on a much greater number of subscribers than its capacity would otherwise allow, perhaps 100 times the equivalent ratio in cellular systems. In addition, the number of subscribers actively listening to a calling channel, i.e., those in the idle mode, already exceeds the number actually involved in conversation by a factor of 20 to 200. As a result, the number of potential subscribers to a dual mode satellite/cellular system can be 2,000 to 20,000 times the satellite system's call capacity.
It is clear that in such a dual mode communication system, the network should preferably know whether a particular mobile phone should be called via the land-based cellular system or via the satellite system. However, to call every subscriber via both systems would pose a very severe calling channel load in view of the potential 100 fold increase in subscribers as mentioned above. Therefore, it is desirable to restrict calling/paging of subscribers via the satellite system to only those mobile phones that are known or suspected to be presently listening to the satellite calling channel. In a cellular communication system or a satellite communication system, it is necessary to restrict paging/calling areas to those areas in which a called mobile phone is thought to be located. Both cellular and satellite systems are more or less global and the paging capacity to call every mobile phone over the whole globe is hard to provide. This problem is solved by means of registration.
Registration means that a mobile telephone informs the network what calling channel it is presently listening to. The network then knows which of a number of limited paging areas the mobile telephone is most likely to be in, and calls to that mobile telephone can be broadcast to that paging area. This process can involve transmitting calls to a mobile telephone via several base station transmitters at different sites in the same paging area. This increases the paging load per transmitter, but reduces the network load for handling reregistrations, as a mobile telephone need no longer transmit a reregistration message every time it finds a stronger transmitter to listen to. In this example, the mobile telephone only needs to reregister when it detects that the transmission it switched to monitoring belongs to a different paging area than the previous one.
The above description of paging areas and reregistration criteria is well known in the art. It is, for example, used in the Pan European GSM cellular system.
To achieve an economically useful capacity to serve a large number of subscribers, satellite communication systems need to allow reuse of the available spectrum many times over the globe. This is achieved by the use of multiple spot beam antennas that divide the illumination of the chosen service area between many smaller regions. Ideally the available spectrum may be reused in each of the smaller regions by use of the invention described in U.S. patent application Ser. No. 08/179,953, entitled "A Cellular/Satellite Communication System With Improved Frequency Re-use", filed Jan. 11, 1994, which is incorporated herein by reference.
The most promising satellite systems for such applications might be considered to be those which are in a near earth orbit and stationary orbit. The disadvantage of satellites in stationary orbits is that huge antennas are needed to create the same size spot beams from the 40,000 km distance and the extra delay in signals traversing that distance creates a problem for two-way conversations. However, the disadvantage of satellites in near earth orbits is that the satellites move and thus the areas that the spot beams illuminate change as the satellites circle the earth. Even if steps are taken to steer the spot beams to more or less the same regions, the satellite will eventually pass over the horizon and will have to be replaced by a rising satellite. When this occurs, it must be avoided that the entire population of mobile telephones listening in the idle mode to a paging channel of the satellite which is about to pass over the horizon attempt to reregister simultaneously the fact that they are now listening to a new paging channel of the new satellite. As mentioned above, the number of mobile telephones in the idle mode is much greater than the satellite system's capacity to handle traffic. Therefore, the problem of bulk registration is difficult to handle.